It seems as though printers are made to run out of toner at the exact moment you need to print five important pages, but Reddit user i_exaggerated unearthed an old tip to force your printer into going a little longer by covering up a sensor bar.

Some printers will lock up and refuse to print once the toner light comes on, but that doesn't mean you can't get a few more good prints out of it. Most toner cartridges have a small sensor hole on the non-gear side of the cartridge, covering this up with a small piece of electrical tape might bring your printer back to life. If the initial forum posts are any indication, you might even get a solid 400-500 extra pages. We tried this on a Brother HL-2040 and it worked right away. It's not going to work for every printer, but it might save you in a pinch.